I tried testing my urls in the test box on the printfriendly.com website and the print preview generated fine. Then I installed the WP plugin and those same pages are missing content on the printable pages.
On one page it's picking up the page title tag, which is not even on the page itself, and then it only picks up one out of the seven content paragraphs on the page. On another pages it doesn't pick up the title (which is fine) and it does grab all the content (which is good!)
So I'm not sure why the plugin would be grabbing oddball content but not the actual content on some pages.